Preformed and induced defense mechanisms provide a wide spectrum of resistance toward numerous pathogens encountered by the plant host. Pathogen specific defense responses are usually initiated by the recognition of a pathogen avirulent (Avr) protein by the corresponding resistance (R) protein of the host. Ultimately, the plant host will produce a series of defense molecules (including pathogenesis-related proteins) to restrict or kill the pathogens. The processes between the initiation of resistance and the production of resistance proteins involve a complex signal transduction network which is yet to be fully elucidated.
In Arabidopsis thaliana, many important hubs of the defense signaling network have been identified by molecular genetic approaches, including EDS1 (Enhanced Disease Susceptibility 1), NPR1 (Non-Expresser of PR Genes 1) and NDR1 (Non Race-Specific Disease Resistance 1). Using similar tactics and together with biochemical studies, the involvement of phytohormone signals in defense responses has been corroborated in A. thaliana, especially the roles of salicylic acid (SA), and the other phytohormones such as jasmonic acid (JA) and ethylene (ET).
Many known signaling strategies are employed in plant defense responses. For instance, some R proteins are receptor kinases while other protein kinases also play significant roles. Biochemical signals such as calcium flux and oxidative burst are also important. Furthermore, there are several reports on the participation of other signaling components such as G-proteins and RING (Really Interesting New Gene) zinc finger proteins.
RING zinc finger proteins are a group of diverse proteins with highly conserved zinc binding domains. Based on the type of cysteine (C) and histidine (H) residue combination, the RING zinc finger domain can be classified into canonical and modified RING zinc fingers. The canonical RING zinc finger can be further grouped into two subclasses: HC subclass (consensus: C—X2—C—X9-39—C—X1-3—H—X2-3—C—X2—C—X4-48—C—X2—C) (SEQ ID NO:1) and H2 subclass (consensus: C—X2—C—X9-39—C—X1-3—H—X2-3—H—X2—C—X4-48—C—X2—C) (SEQ ID NO:2) (Stone, S. L., et al., Plant Physiology (2005) 137:13-30). Modified RING zinc fingers include RING-C2, RING-v, RING-D, RING-S/T and RING-G.
Many members of the RING zinc finger protein family (including both HC and H2 subclasses) are E3 ubiquitin ligases. Different subclasses of the RING zinc finger domain determine specificity toward different E2 ubiquitin conjugating enzymes. Other RING zinc finger proteins can bind to nucleic acids or interact with other protein targets. Besides the ubiquitin mediated degradation pathway, RING zinc finger proteins also play important roles in organelle transport and transcription/translation regulations.
In rice, more than 30 resistance loci (Xa loci) against the pathogen Xanthomonas oryzae pv. oryzae (Xoo) have been identified and 6 Xa genes were cloned mainly by map-based cloning approaches. Several pathogenesis-related (PR) genes have been reported to contribute directly to the resistance mechanism. However, only a few key components of the signal transduction pathway from the onset of R protein-Avr protein interaction to the actual resistance development have been studied. To obtain new signal transduction components related to Xoo resistance in rice, cDNA clones differentially expressed in rice lines harboring Xa loci were searched for.
The present inventors have cloned and characterized a novel RING zinc finger protein gene (OsRHC1) from rice. OsRHC1 is differentially expressed under wounding in near isogenic lines containing the Xa14 or Xa23 resistance loci, but not in the corresponding susceptible recurrent parents. Ectopic expression of OsRHC1 in transgenic A. thaliana enhances its resistance toward bacterial pathogens and such protective function depends on the action of the 26S proteasome.